Mass spectrometers may include an ion trap where ions are stored either during or immediately prior to mass analysis. The achievable high performance of all trapping mass spectrometers is known to depend most critically on the quality of the electromagnetic fields used in the ion trap, including non-linear components of higher orders. This quality and its reproducibility are defined, in their turn, by the degree of control over imperfections in manufacturing the ion trap and the associated power supplies that provide signals to electrodes in the ion trap to create the trapping field. More complex assemblies are known to have greater difficulties in achieving required levels of performance because of larger spreads or accumulation of tolerances and errors, as well as increasingly troublesome tuning of the trapping field.
This problem is exemplified for the Orbitrap mass analyser, such as that described in U.S. Pat. No. 5,886,346. In this Orbitrap mass analyser, ions are injected in pulses from an external source such as a linear trap (LT) into a volume defined between an inner, spindle-like electrode and an outer, barrel-shaped electrode. Exceptional care is taken with the shape of these electrodes so that together their shapes can create as ideally as possible a so-called ‘hyper-logarithmic’ electrostatic potential in the trapping volume of the form:
      U    ⁡          (              r        ,        z            )        =                    k        2            ⁢              (                              z            2                    -                                    r              2                        2                          )              +                  k        2            ⁢                        (                      R            m                    )                2            ⁢              ln        ⁡                  [                      r                          R              m                                ]                      +    C  where r and z are cylindrical co-ordinates, C is a constant, k is the field curvature, and Rm is the characteristic radius. The centre of the trapping volume is defined to be z=0 and the trapping field is symmetric about this centre.
Ions may be injected into the Orbitrap in various ways (either radially or axially). WO-A-02/078,046 describes some desirable ion injection parameters to ensure that ions enter the trapping volume as compact bunches of a given mass to charge m/z ratio, with an energy suitable to fit within the energy acceptance window of the Orbitrap mass analyser. Once injected, the ions describe orbital motion about the central electrode, with axial and radial trapping within the trapping volume achieved using static voltages on the electrodes.
The outer electrode is typically split about its centre (z=0), and an image current induced in the outer electrode by the ion packets is detected via a differential amplifier. The resultant signal is a time domain ‘transient’ which is digitised and fast Fourier transformed to give, ultimately, a mass spectrum of the ions present in the trapping volume.
The gap splitting the outer electrode may be used to introduce ions into the trapping volume. In this case, ions are excited to induce axial oscillations in addition to the orbital motion. Alternatively, the ions may be introduced at a location displaced along the axis from z=0, in which case the ions will automatically assume an axial oscillation in addition to the orbital motion.
The precise shape of the electrodes and the resultant electrostatic field result in ion motion which combines axial oscillations with rotation around the central electrode. In an ideal trap, the hyper-logarithmic field does not contain any cross-terms in r and z such that the potential in the z direction is purely quadratic. This results in ion oscillations along the z-axis that may be described as an harmonic oscillator, independent of the ions' (x, y) motion. In this case, the frequency of the axial oscillations is related only to the mass to charge ratio (m/z) of ions as:
  ω  =            k              m        /        z            where ω is the frequency of oscillation and k is a constant.
The high performance and resolution required places a high requirement on the quality of the field produced in the trapping volume. This in turn places a high requirement on perfecting the shape of the electrodes. It is perceived that any deviations from the ideal electrode shape will introduce non-linearities. This results in the frequency of axial oscillations becoming dependent upon factors other than purely the mass to charge ratio of the ions. The consequence of this is that factors such as mass accuracy (peak position), resolution, peak intensity (related to ion abundance) and so forth may be compromised, possibly to the extent of becoming unacceptable. Mass production of the electrode shapes to such an exacting tolerance, therefore, is a challenge.
The Orbitrap mass spectrometer is only a particular case of a more general class of substantially electrostatic multi-reflection systems which are described in the following non limiting list: U.S. Pat. No. 6,013,913, U.S. Pat. No. 6,888,130, US-A-2005-0151076, US-A-2005-0077462, WO-A-05/001878, US-A-2005/0103992, U.S. Pat. No. 6,300,625, WO-A-02/103747 or GB-A-2,080,021.